1. Field of the Invention
The invention relates to an optical display device having a large number of individual light sources distributed at the nodes of a regular lattice in a matrix arrangement. It is particularly suitable for use in large size (possibly beyond 10 m.sup.2) for location in the open air or in front of buildings.
2. Description of the Prior Art
Optical display devices using a matricial arrangement of LEDs and switching circuits have already been proposed. An apparent ultimate in simplicity and ingenuity is achieved in a device (German 2,942,777) where a series arrangement of a LED and a switching element is connected between a row line and a column line. The switching element is of a type which becomes electrically conductive when the voltage across its terminals exceeds a predetermined value and remains conductive until that voltage has decreased under a second predetermined value, lower than the first. The row lines and the column lines are connected to respective interfaces controlling the voltages on the line. However, that approach has a number of drawbacks which render it pratically useless. Each of the interfaces should be constructed for controlling the voltages on the lines at anyone of three different values. The power necessary for operation of the LEDS is transferred on the lines and consequently it is almost impossible to maintain the three voltages required for operation at the necessary level of stability throughout the length of the line whatever the number of LEDs which are energized.
Similar comments are applicable to the other devices in which the light source is controlled and energized by a row line and a column line only, for instance those described in German 2,149,688 (Secretary of State . . . ,) and the Marconi Review, Vol. 35, No. 184, 1972 , pp. 24-41 "Chalcogenide switching devices".